Meters for metering the various forms of electrical energy are well known. Utility company meters can be of three general types, namely, electromechanical based meters (output generated by a rotating disk), purely electronic component based meters (output component generated without any rotating parts) and a hybrid mechanical/electronic meter. In the hybrid meter, a so-called electronic register is coupled, usually optically, to a rotating disk. Pulses generated by the rotating disk, for example by light reflected from a spot painted on the disk, are utilized to generate an electronic output signal.
It will be appreciated that the use of electronic components in electric energy meters has gained considerable acceptance due to their reliability and extended ambient temperature ranges of operation. Moreover, contemporary electronic signal processing devices, such as micro controllers, have a greater accuracy potential for calculating electrical energy use than prior mechanical devices. Consequently, various forms of electronic based meters have been proposed which are virtually free of any moving parts.
Examples of electronic meters are disclosed in U.S. Pat. Nos. 5,548,527--Hemminger et al. and 5,555,508--Munday et al., incorporated herein by reference. In those meters, signal processing has been generally distributed between a digital signal processing integrated circuit device and a microcontroller device. As will be apparent, clock signal generation is necessary for the operation of such devices. Indeed, for applications in which such electronic devices are used to monitor time-based parameters, such as time-of-use metering, the accuracy of such clock signals can have a significant impact on the accuracy of the monitoring data. In metering applications, clock signals are typically generated in two ways, namely, in relation to the line frequency or through the use of an internal oscillator.
In the United States, quality clock signals can be generated in relation to line frequency, i.e., the frequency of the voltage signal being supplied to a given customer, which is 60 Hz. It is widely known that the frequency of the U.S. power grid is extremely stable over long periods of time. Internationally, the reliability of line frequency based clock signals is inconsistent, particularly in third world countries. In such environments, some other method is required for monitoring time, such as internal oscillators.
In view of the above, an electronic based energy meter will have maximum salability if it can monitor time accurately in all environments, i.e., environments in which grid frequency is stable and environments in which grid frequency is unstable. Moreover, in time-of-use meters, the measurement of real time must be maintained at all times, even during power outages. To these ends, the meters described in the above patents incorporate two crystal oscillators, one of the oscillators being used to measure real time during power outages, i.e., low power battery operation.
The accuracy of a crystal oscillator is its ability to generate a consistent signal over time. This characteristic is described in terms of PPM (parts per million). A 1 MHZ crystal which has an error of .+-.10 PPM will generate between 999,990 and 1,000,010 pulses every second. Since in metering applications, customer billing is based on measured time, accuracy is very important. Presently, many utility companies desire accuracies of 5 PPM.
One method for ensuring accurate crystal oscillator signals is to purchase commercially available oscillators having the desired accuracy characteristics. Unfortunately, such oscillators are relatively expensive and for high volume products such as energy meters, such expense becomes economically prohibitive. Moreover, during power outages, meters such as those described in the above patents, go into a low power mode. 32.768 kHz crystal oscillators have been used for such low power operations because they are inexpensive and well suited for low power operation. Unfortunately, the initial and long term accuracy of such crystals is commonly 50 PPM or greater. Obtaining such an oscillator with the desired accuracy specifications is also cost prohibitive.
Consequently, a need still exists for an electronic meter which incorporates accurate, low cost, crystal oscillators.